Do “deeply buried” HRV ducts need a vapor barrier?
I’ve got an ERV install going into a retrofit remodel situation (climate zone 4C PNW). Some of the ductwork can be located within conditioned space, however around 40% of both the exhaust and supply trunk ducts must run through an unconditioned attic.
To be clear, I am only discussing the ducts which supply and pull air from within the house, not the ducts connecting the ERV to the exterior.
These trunk ducts are galvanized, very well sealed with mastic. I want to discuss the insulation strategy for them. Of course they should be insulated being in an unconditioned attic. They are on the bottom of the attic, which will receive blown cellulose to R-49.
When heat pump ductwork is deeply buried, it must be insulated and have a vapor barrier to keep moist attic air from condensing on the cold duct. Of course the supply temperature of the ductwork is very cold making this necessary. However for a ERV/HRV duct, the duct temp should always be close to the living space temperature. In this case, I don’t see a need for the vapor barrier. Thus it seems simply leaving the galvanized ductwork bare, and blowing insulation around and above it should be sufficient to insulate it.
Is this a correct analysis? What does code have to say about this?
Thanks,
Evan
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Replies
On a hot day, (high attic temps) while running AC, your duct surface temps may very well be below the attic air dew point, with the knowledge that some air movement will occur in cellulose. Also, depending on the duct size, R value will be less at the top of the duct. Allowing air to contact the cool surface would likely result in condensation which in turn would wet the cellulose.
I can't speak to code on this, but in a larger commercial project (7A) all of our attic galvanised duct (also buried in cellulose) was insulated and taped carefully before the cellulose was blown in. Having duct in an attic is something you'd like to avoid, but in this case it was a 2nd floor with an eight foot ceiling over about 4500 square feet. It added a lot with respect to cost to the project.
I appreciate the advice, but I'm still having a tough time seeing how that situation could occur. In the summer, the exhaust air will be equal to or slightly warmer than the room temperature, say 72 degrees. And the supply air will be a bit warmer, as ERV's aren't 100% efficient. Say 75 degrees.
The record dew point in this region is 65 degrees. Since this is a fully separately ducted unit, I don't believe the air temperature in the ducts could get below room temperature, thus should never condense.